Explain how ionic bonds form by transfer of electrons and covalent bonds by sharing, predict which forms from the elements involved, and relate bond type to properties (MA STE HS-PS1-2, bonding from electron states).

What this topic is asking

Standard HS-PS1-2 asks you to explain the outcome of a chemical reaction from the outermost electron states of the atoms. The starting point is bonding: atoms join because doing so lets their valence electrons reach a stable, full-shell arrangement. Massachusetts high school chemistry expects you to describe how ionic and covalent bonds form, predict which one forms from the elements involved, and connect the bond type to the properties of the substance.

Why atoms bond

Atoms bond because a bonded arrangement is more stable (lower in energy) than separate atoms. The driving idea is the octet rule from Module 1: atoms gain, lose, or share electrons to reach a full outer shell like a noble gas. How they do it depends on the elements:

• A metal has few valence electrons and loses them easily, so it tends to give electrons away.
• A nonmetal has many valence electrons and attracts more, so it tends to take or share electrons.

This sets up the two main bond types.

Ionic bonding

When sodium meets chlorine, sodium (1 valence electron) loses it and chlorine (7 valence electrons) gains it. Sodium becomes $\text{Na}^+$ and chlorine becomes $\text{Cl}^-$, and the opposite charges attract. The ions do not pair off in isolation; they stack into a giant repeating ionic lattice, with each ion surrounded by ions of opposite charge. The compound's formula (NaCl) gives the simplest whole-number ratio of ions, not a molecule.

The strong lattice of attractions explains the properties: ionic compounds have high melting and boiling points (much energy is needed to pull the lattice apart), they are brittle (knocking the layers so that like charges align makes them repel and shatter), and they conduct electricity when molten or dissolved but not as solids, because the ions must be free to move to carry charge.

Covalent bonding

Two nonmetals cannot both lose electrons, so instead they share. In a water molecule, oxygen shares electrons with two hydrogen atoms; in an oxygen molecule, the two atoms share a double bond. Covalent bonding usually produces small, discrete molecules (or sometimes giant covalent networks like diamond). Molecular substances have lower melting and boiling points than ionic compounds, because the weak forces between separate molecules (not the strong bonds within them) are what melt or boil; and they usually do not conduct electricity, because there are no free charged particles.

Predicting the bond type

The fastest rule for a first chemistry course:

• Metal + nonmetal gives an ionic bond (electron transfer).
• Nonmetal + nonmetal gives a covalent bond (electron sharing).
• Metal + metal gives a metallic bond, covered in metallic bonding and material properties.

A more precise test uses electronegativity difference: a large difference favors ionic, a small difference favors covalent, and an intermediate difference gives a polar covalent bond. That refinement is developed in molecular geometry and polarity.

Try this

Q1. Predict the bond type in a compound of potassium and bromine. [1]

• Cue. Ionic (potassium is a metal, bromine a nonmetal, so electrons are transferred).

Q2. Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. [2]

• Cue. In the solid the ions are locked in the lattice and cannot move; when molten the ions are free to move and carry charge.